Method for cleaning and drying semiconductors

ABSTRACT

A semiconductor wafer is cleansed of loose foreign surface matter and chemical impurities near the surface in an apparatus which passes superheated steam over the wafer. Condensate is permitted to form and drip off the wafer. After rising above 100° C. the wafer becomes dry, and is removed from the apparatus and then permitted to cool.

This is a division of application Ser. No. 726,058, filed Sept. 23,1976, which issued on Mar. 21, 1978 as U.S. Pat. No. 4,079,522.

This invention is generally related to methods for cleaning and dryingsemiconductor wafers before subsequent processing into integratedcircuits and other devices. It is specifically related to methods forcleaning a semiconductor material such as silicon, gallium arsenide,germanium or the like with superheated steam using controlled variationsin the temperature of the semiconductor.

In the process of manufacturing an integrated semiconductor circuit orsemiconductor device using a semiconductor wafer various impurities arediffused therein and layers of materials such as metallization layersare formed thereon. It is necessary prior to these steps to free thesurface of solid matter, films of chemical impurities and solvents whichmay have been absorbed or adsorbed during prior cleansing and processingsteps. For example, usually, during the manufacture of semiconductordevices, the semiconductor wafer is cleaned in alcohol to remove anycontaminants which may be on the surface of the wafer. Sometimes a waferabsorbs some of the alcohol. One method which has been used to removethis material is the passage of distilled water over the surface of thewafer.

Also, sometimes alcohol is used as a final solvent and cleansing agentand the semiconductor wafers are stored therein prior to processing thewafer into operable devices.

FIGS. 1(a) and 1(b) illustrate an embodiment of a novel apparatus whichmay be used to carry out steps of the novel method; and

FIGS. 2(a) through 2(e) illustrate steps in the novel method.

Superheated steam is water vapor at a higher temperature than 100° C.Superheated steam is produced when steam as water vapor is lead awayfrom boiling water and heated.

The novel apparatus for the method is illustrated in FIG. 1(a) whereinthere is shown a chamber 10 for receiving a semiconductor wafer 11. Thechamber 10 is comprised, for example, of quartz and has an input opening12 and output opening 14 for the input and exhaust, respectively, ofsuperheated steam. A large quartz tube having an aperture 16 extendsperpendicular to the chamber 10. See FIG. 1(b). The aperture 16 providesa passageway for placing the wafer inside the chamber 10. A heating coil18 is wrapped around one portion of the chamber 10 and is connected toan electrical power source 20. The source 20 connected with the heatingcoil 18 heats a portion of the inside of the chamber 10. Similarly, aheating coil 22 wrapped around another portion of the chamber 10 andconnected to another electrical power source 24 provides a means forheating this portion of the chamber 10. Both coils 18 and 22 are made ofelectrically conductive metal wire. The sources 20 and 24 may beradiofrequency power sources.

The sources 20 and 24 and their associated coils 18 and 22 are used tomaintain the inside of the chamber 10 at a specified temperature, forexample, 105° to 160° C. This insures that steam at a temperature above100° C. passes over the wafer 11. The heater coils 18 and 22 are turnedon and permitted to heat the chamber before the cleaning operationbegins.

A holder 26 is provided and adapted for holding the wafer 11. The holder26 (adapted for cooling by flow of a fluid therethrough) is comprised ofa hollow low thermal mass of quartz, for example, and has a quartz inputtube 30 connected thereto on one side and another quartz output tube 32connected to another side. A fluid pump 34 is connected to the inputtube and delivers the fluid, for example, water, to the holder 26. Thefluid is pumped from a reservoir 36 connected to the pump 34. The fluidis exhausted through the exhaust tube 32. The holder 26 has a passageway(not shown) therethrough which interconnects the tube 30 to the tube 32providing for the passage of fluid through the holder 26 to cool theholder 26. Grommets 38 comprised of a high temperature polymer, forexample Teflon, seal the connection between the chamber 10 and the tubes30 and 32 exiting therefrom. The seal, the chamber 10, and the tubes 30and 32 may be improved by means of a silicone grease or other hightemperature lubricant placed at the connection.

FIG. 1(b) is a cross-sectional sideview of the chamber 10 with theholder 26 therein. Steam enters the chamber 10 through the opening 12.The steam is exhausted through the other opening 14 after passing overthe wafer 11. The aperture 16 is an access way which is sealed duringthe cleaning operation and opened to remove the wafer 11 upon completionof the cleaning steps. An orifice 40 is in the holder 26. The orifice 40is connected to the tube 32.

The temperature is controlled by regulating the electrical powersupplied to the coils 18 and 22. The coils 18 and 22 are shown incross-section wrapped around portions of the chamber 10 above and belowthe orifice 16.

The chamber 10 may be approximately 15 inches in length and 6 inches indiameter, for example. The orifice 16 may be approximately 6 inches indiameter, for example, and extend approximately 4 inches away from thechamber 10, for example. The electrical power sources 20 and 24 maycomprise variable voltage current sources, for example variabletransformers, i.e., electrical devices which provide alternating currentat various voltages. Heat is produced by passing alternating currentthrough the wire coils 18 and 22 at a preselected voltage. The amount ofvoltage may be directly correlated to the heat produced from the coils18 and 22 to control and cycle the temperature in the chamber 10.

The superheated steam for input into the opening 12 of the chamber 10may be provided, for example, by a steam superheater (not shown). Asuperheater is a heat transfer surface, tubular in character, forexample arranged to receive saturated steam at the inlet of the tubesand to deliver superheated steam steadily at the outlet. Heat isreceived from a combustion of fuels, for example, by direct radiation tothe surface of the superheater tubes or by convection from hot gases,for example, made to flow over the tubes' surfaces, or both. Thesuperheater tubes are made small in diameter to promote maximum contactof the steam with the hot tube wall.

In the operation of this apparatus an inclined surface 42 of the holder26 shown in FIG. 1(b) is cooled. This is done by pumping water throughthe holder 26. The rate at which the water is pumped through the holder26 can determine the rate of cooling (or heating) of the holder 26 andthereby the temperature of the wafer 11 on it. Any power pump may beused for the fluid pump 34 shown in FIG. 1(a). The rate of fluid flowthrough the holder 26 is determined by the rate of operation of the pump34 which rate may be controlled by the amount of electric power suppliedto the pump 34. Variable rate power pumps are known.

The method of the invention is illustrated by FIGS. 2(a) through 2(e).FIG. 2(a) shows a semiconductor wafer 11 at a temperature T equal to aninitial temperature, room temperature, for example. The initial orstarting temperature in this method may range from above 0° C. to lessthan 100° C.

The wafer 11 is placed on the holder 26 of the apparatus (see FIG. 1(b))and steam is passed over the wafer (FIG. 2(b)) while the wafer ismaintained at a temperature of less than 100° C. This temperature T<100°is maintained by passing water through the holder 26 using the fluidpump 34. As steam is passed over an inclined surface of the wafer 11, acondensate 44 forms on the surface thereof. The action of thesuperheated steam passing over the wafer 11 also heats the wafer 11 andabsorbs various impurities which have been previously diffused in thesurface, as well as dissolves impurities on the surface of the wafer 11.

Inclining the wafer 11 at an angle away from the vertical such thatcondensate 44 formed on the inclined surface facing the input ofsuperheated steam washes impurities down the substrate preventsrecontamination of upper portions of the surface. The wafer 11 becomesprogressively cleaner from top to bottom during this part of theprocess.

Once sufficient condensate 44 has formed on the surface to thoroughlyclean the wafer 11, the fluid pump 34 of FIG. 1(a) is shut off and thewafer 11 is permitted to rise to a temperature T above or equal to 100°C. Typically about 5 to 120 minutes is required to thoroughly clean awafer. However, actual times will vary and will depend upon the amountand type of contaminants in and on the wafer. These times may bedetermined by simple trial and error sample processing of a few wafersfrom a group or by repetition of the novel cleaning steps forpreselected time increments.

FIG. 2(c) shows superheated steam continuing to pass over the wafer 11.The temperature T of the wafer 11 is approximately equal to 100° C. andthe condensate 44 drips off the surface.

As illustrated in FIG. 2(d), once the temperature T of the wafer 11 isgreater than 100° C. the superheated steam merely passes over the wafer11 and no condensate is formed. The fluid pump 34 is then shut off andthe wafer is raised to a temperature above 100° C., permitted to dry fora period of 3 to 5 minutes and removed from the chamber 10. All watervapor and other substances with a boiling point lower than that of thetemperature T of the substrate are evaporated or boiled off aftermaintaining the temperature T greater than 100° C. for a sufficientperiod of time, such as the 3 to 5 minute period disclosed here.

In FIG. 2(e) the wafer 11 is shown removed from the superheated steamenvironment to a cool environment where the temperature of the wafer 11becomes greater than or equal to the initial temperature but typicallyless than 100° C. (The wafer 11 is withdrawn from the chamber 10 whileat a temperature greater than 100° C. Because the wafer 11 is withdrawnwhile above 100° C. to an environment below 100° C., it is dry whenremoved.)

I claim:
 1. A method for cleaning and drying a surface of asemiconductor wafer comprising:(a) providing a semiconductor wafer at aninitial temperature below 100° C. and above 0° C.; (b) placing the waferagainst a holder having a low thermal mass, the holder having aninclined surface adapted to receive the wafer and adapted for thepassage of cooling fluid therethrough; (c) passing superheated steamover a surface of the wafer; (d) condensing a portion of the superheatedsteam on the surface whereby a water condensate is formed; (e) drippingthe condensate from the inclined wafer for a preselected time; (f)raising the temperature of the wafer above 100° C.; and (g) removing thewafer to an environment below 100° C. after all water adjacent the waferis evaporated.